The present invention relates to a method and apparatus for digitally measuring rotational movements of two reference systems relative to each other through the use of two light beams circulating in opposite directions and the detection of phase and frequency differences in the output of light sensors mounted on one of such reference systems.
Methods and apparatus of this general type are known in various embodiments, including those previously disclosed by the applicant. They are based on the principle of measuring transit time differences in two reference systems which rotate with respect to one another, one forming the measuring system and the other forming the system to be measured. Numerous types of laser gyros and gravitational wave detectors and similar devices are based on this idea. In all these devices it is found, however, that the measurement of low rotational speeds with high a precision and resolution not only requires high expenditures with respect to equipment, but also is characterized by difficulties with respect to manufacturing techniques. Thus, for example, direct-current tacho generators furnish very low voltages at low speeds and, in addition, the ripple of the signal, as a result of the commutation and the transition from one commutator bar to the adjacent one, has a distorting effect. Similarly, devices for measuring rotational speed by means of digital angle generators require a very high resolution for the detection of low speeds, which results in increased expenditures with respect to the weight and the overall size.
It is an object of the present invention to provide a method and apparatus for the measurement of angular movement which minimizes the overall size and weight of the measuring device.
Another object of the invention is to provide a device for measuring angular movement which is simplified without any impairment of the reliability and the precision of the measuring system, especially in the case of low rotational speeds.
These and other objects of the invention are achieved by providing a source of two light spots which rotate in opposite directions in concentric orbits in a measuring plane. Sensors are mounted on a member which is rotatable about the axis of rotation of the light spots, one such sensor being disposed adjacent the orbit of each of said light spots. The angle and speed of rotation of such member are then determined by measuring the phase or frequency of the output signals from the sensors.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.